Investments - Additional Information (Detail)	1 Months Ended	3 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($)
Schedule of Debt and Equity Securities [Line Items]
Principal amount of bonds and certificates of deposit		$ 296,100,000
Securities Exceeding 10% of Shareholders' Equity	$ 0	0	$ 0
Non-Income Producing Securities		0
Open reverse repurchase commitments	0	0	0
Securities Sold under Agreements to Repurchase	0	$ 0	0
Debt Securities, Available-for-sale, Unrealized Loss Position, Positions Downgraded | security		3
Combined fair value of securities		$ 31,900,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Downgrades in Period		700,000
Debt Securities, Available-for-sale, Allowance for Credit Loss, Not Previously Recorded		0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0	$ 0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff		$ 0
Percent Change in Investment Income		9.00%
Percent Change in Book Yield		22.00%
Proceeds from Debt, Net of Issuance Costs	$ 1,000,000,000.0